The present invention relates to an exhaust emission control system for an internal combustion engine, and more particularly to such an exhaust emission control system having a function of monitoring the concentration of sulfur oxide (SOx) contained in exhaust gases.
Conventionally known is a technique for removing NOx (nitrogen oxide) contained in exhaust gases by providing a NOx removing device including a NOx absorbent in an exhaust system of an internal combustion engine. Further, a technique of determining a degree of deterioration of the NOx removing device is known in the art (Japanese Patent Laid-open No. Hei 10-299460). In this technique, two oxygen concentration sensors are arranged upstream and downstream of the NOx removing device, and the air-fuel ratio of an air-fuel mixture to be supplied to the engine is changed from a lean region to a rich region with respect to the stoichiometric ratio. Then, the degree of deterioration of the NOx removing device is determined according to a delay time period from the time an output from the upstream oxygen concentration sensor has changed to a value indicative of the rich air-fuel ratio to the time an output from the downstream oxygen concentration sensor has changed to a value indicative of the rich air-fuel ratio.
However, when the concentration of SOx in the vicinity of each oxygen concentration sensor becomes high, an output characteristic of the oxygen concentration sensor changes. Accordingly, when the concentration of SOx in the vicinity of each oxygen concentration sensor is high, there is a case that the deterioration determination of the NOx removing device cannot be accurately performed. When the degree of enrichment of the air-fuel ratio is low, that is, when the air-fuel ratio after enrichment is near the stoichiometric ratio, the effect of SOx tends to become especially remarkable.